A known electric storage device includes an electrode assembly and an electrolyte held in a case. The electrode assembly includes electrode plates. Each electrode plate has a metal foil surface and an active material layer formed on the metal foil surface. Such an electric storage device may be a battery or an electric double layer capacitor. The case may include a safety valve in the outer surface of the case to exhaust gas in the case if an internal pressure in the case reaches a specific value. The safety valve may include a breakable portion. A protective film may be affixed to the outer surface of the case so as to cover the safety valve.
With the protective film, foreign substances, such as water and oil, are less likely to adhere to the safety valve, that is, the safety valve is protected from corrosion. However, the protective film may cause the following problem. Enclosed space is provided between the protective film and the safety valve and thus an internal pressure in the enclosed space increases during breaking of the breakable portion. Namely, force against the increasing internal pressure needs to be applied to break the breakable portion.
If a gas discharge outlet such as the safety valve is covered with the protective film, the gas discharge outlet is not easily broken even when the internal pressure reaches a specific level. Furthermore, the protective film may be removed from the case when gas is discharged and the removed film may have an adverse effect on peripheral devices.